Download sheep brain dissection

SHEEP BRAIN DISSECTION
The sheep brain is anatomically similar to the human brain, so it serves as an excellent
model for dissection. Dissection of the meninges must be completed before making
observations of the brain.
DISSECTING THE SHEEP MENINGES
1. Obtain a sheep brain and a set of dissecting tools
2. The dura mater is the tough, opaque outer covering of the brain. Carefully cut it
away without damaging underlining structures by lifting the dura mater away
from the surface of the brain with forceps and cutting the membrane with scissors.
Begin at the posterior end of the brain and cut just to the left or right of the
midsagittal plane along the superior surface.
3. The dura mater has two layers: an outer layer that is attached to the inside of the
cranial bones and an inner layer that forms the brain covering.
4. It is very difficult to remove the dura mater from the inferior surface of the brain
without damaging the origins of the cranial nerves. Do not be concerned if this
occurs; however, attempt to save the two optic nerves and the optic chiasm.
5. The somewhat glossy surface of the sheep brain is the thin pia mater. Notice that
it follows the complicated pattern of convolutions and grooves along the surface
of the cerebrum.
DISSECTING SURFACE STRUCTURES ON THE SHEEP BRAIN
1. Identify the cerebrum, which consists of two cerebral hemispheres. Note that the
hemispheres are separated by the longitudinal fissure.
2. Notice that the surfaces of the hemispheres are characterized by a complex system
of raised gyri. The gyri are separated by shallow sulci or deep fissures.
3. Identify the following cerebral lobes.
a. The frontal lobes are located at the anterior end of the cerebrum.
b. The parietal lobes are just posterior to the frontal lobes. In each
hemisphere, the frontal lobe is separated from the parietal lobe by the
central sulcus. The sulcus is difficult to identify on the sheep brain.
c. The occipital lobes are located at the posterior tip of the cerebrum. Each
occipital lobe is separated from its corresponding parietal lobe by the
parieto-occipital sulcus (also hard to identify).
d. The temporal lobes are located along the lateral aspects of each
hemisphere. Identify the lateral fissure, which separated the temporal
lobe from the frontal lobe.
4. Carefully separate the two cerebral hemispheres of the sheep brain with your
hands and look down in to the longitudinal fissure. By doing this, you will
identify the corpus callosum, which contains myelinated fibers that connect the
hemispheres.
5. Identify the cerebellum, just posterior to the cerebrum. Note that the cerebellum
is also divided into cerebellar hemispheres.
6. Gently separate the cerebrum and cerebellum at the transverse fissure and
identify two pairs of prominent tissue elevations on the roof of the midbrain. The
anterior pair is called the superior colliculi and the posterior pair is the inferior
colliculi. Collectively, all four are called the corpora quadrigemina.
7. Observe the inferior surface of the brain and identify the following structures.
a. The olfactory bulbs are ovoid masses on the inferior surface of the frontal
lobe. They give rise to olfactory nerves.
b. The optic chiasm, where the two optic nerves converge, is anterior to the
pituitary gland. At this location, some fibers of each optic nerve cross to
the opposite side.
c. The pituitary gland, located posteriorly to the optic chiasm, may be
absent.
d. On the brainstem, the elevated mass of tissue on the ventral surface is the
pons.
e. The medulla oblongata is the column of tissue that extends posteriorly
from the pons and is directly continuous with the spinal cord.
EXAMINING A MIDSAGITTAL SECTION OF THE SHEEP BRAIN
1. Place the sheep brain on the dissecting tray with the dorsal surface up. Using a
knife, make an incision along the midsagittal plane of the brain. Cut along the
longitudinal fissure in an anterior to posterior direction, staying as close to the
midline as possible. When you reach the posterior end of the cerebrum, continue
your midsagittal incision through the cerebellum and along the length of the
brainstem. From this view, the medial surface of the cerebral hemisphere can be
seen.
2. Locate the corpus callosum, which is the arching band of white matter located
just inferior to the cerebral hemisphere. Identify the cingulated gyrus which
curves over the corpus callosum. This is part of the limbic system where
emotions and other related behaviors are regulated.
3. Just inferior to the corpus callosum is a thin membrane called the septum
pellucidum. This membrane forms a barrier between the two lateral ventricles.
Under the septum pellucidum you will find the lateral ventricle.
4. Identify the thalamus, a large circular region inferior to the corpus callosum. The
right and left thalami form the lateral walls of the third ventricle.
5. The region just inferior to the thalamus is the hypothalamus.
6. Locate the mamillary body, a nucleus in the hypothalamus that is part of the
limbic system. It serves as a control center for motor reflexes associated with
eating.
7. Identify the fornix, a gently curving fiber tract that runs between the corpus
callosum and the thalamus. The fornix connects the mamillary body to the
hippocampus, part of the limbic system, important for memory but located deep
within the temporal lobe.
8. Identify the pineal body, small gland wedged between the thalamus and
midbrain.
9. Immediately posterior to the cerebrum is the cerebellum. Note the presence of the
grey matter along the outer surface of the cerebellum, the cerebellar cortex. A
region of white matter, the arbor vitae, is deep to the cortex.
10. The midbrain is directly posterior to the thalamus and hypothalamus. Superiorly,
the midbrain is dominated by the superior and inferior colliculi. A fiber tract,
known as the cerebral peduncle, passes inferiorly. Traveling between the
colliculi and cerebral peduncle is the cerebral aqueduct, a narrow passageway
that connects the third and fourth ventricle.
11. The pons is immediately posterior to the midbrain, followed by the medulla
oblongata. The fourth ventricle is the cavity located between the cerebellum,
superiorly, and the pons and medulla oblongata, inferiorly.
EXAMINING A CORONAL SECTION OF THE SHEEP BRAIN
1. Obtain one half of your midsagittal incision
2. Using a knife, make three coronal slices. Depending on where you slice, you will
see different views of structures.
3. Make the first slice through the frontal cortex and the corpus callosum.
4. Examine your coronal section of the brain and identify the following structures.
a. The cerebral cortex, a thin layer of grey matter on the surface of the
cerebrum
b. The white matter, deep to the cerebral cortex
c. The basal nuclei, a region of grey matter deep within the cerebrum
(caudate nucleus, and putamen)
d. The two lateral ventricles
e. The internal capsule, a projection fiber tract that travels between the
thalamus and basal nuclei
f. The corpus callosum, the main commissural fiber tract that travels
between the two cerebral hemispheres
First slice: through anterior portion of corpus callosum
Second slice: through mass intermedia of thalamus
Third slice: through mass intermedia of thalamus
Name: _______________
Brain Dissection Questions
Period: _________________
Self-Test:
A. From the information above and in your text, complete the following statements about spinal cord
structure and about the structure of the brain.
1. The spinal cord lies within a body cavity known as the _____________________.
2. The meningeal layer that must be penetrated first during a spinal tap, in which a needle is
penetrated to the subarachnoid space to withdraw cerebrospinal fluid, is the
_______________________________________________________________.
3. The _____________________________ are the two ventral arms of gray matter that contain
motor functions.
4. In the spinal cord the _________________ matter is located in the center.
5. The ____________________ consists of white matter that occupies the dorsal side of the cord.
6. The thin, delicate __________________ adheres to the outer surface of the brain and is continuous
with the convolutions.
7. The upfolds in the cerebral convolutions are called ________________________.
8. The ________________consists of white transverse fibers that connect the right and left
hemispheres of the cerebrum.
9. The largest structure of the diencephalons is the ___________________________.
10. The third ventricle communicates with the lateral ventricles by way of a narrow channel known as
the _______________________________________________.
11. The pituitary gland is attached to the _____________________ by a short stalk.
12. The tectum and cerebral peduncles are components of the __________________.
13. The _______________ contains narrow convolutions and lies ventral and posterior to the
cerebrum.
14. The pyramids and numerous reflex centers characterize the
_________________________________________________________________.
15. The ___________________ is comprised of the midbrain, pons, and medulla.
C. Match the terms associated with the central nervous system in Column I with their corresponding
characteristics in Column II.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
____ posterior gray horns
____ lateral funiculus
____ arachnoid
____ longitudinal fissure
____ sulci
____ arbor vitae
____ hypothalamus
____ optic chiasma
____ cerebral peduncles
____ third centricle
____ medulla oblongata
____ hindbrain
____ dura mater
A. furrow between hemispheres
B. “tree of life”
C. ventral region of the brain
D. white matter in the spinal cord
E. component of the midbrain.
F. unites with the spinal cord.
G. minor grooves in convolution
H. cavity in the brain
I. tough outer meningeal later
J. sensory area of spinal cord
K. crossing of optic nerves
L. delicate middle meanings
M. center of involuntary functions